Hunted (Plutonia Experiment level 11) was frightening, definitely. But this level can make you fear even more. Where are those laughing archviles? Where are all those enemies teleporting? What are those metallic steps? There are traps everywhere you go, and some very unique areas. See if you can chase that cyberdemon to his death, or yours? This dark base is occupied with so many enemies that you might encounter problems when trying to save the game in the first room. If that happens, just save after the first room.
Upon leaving the very first room a door is triggered and stays up for some seconds down the winding corridor (in the 4th bend). Behind it is a teleporter that leads to a hidden area with stairs and quite some ammo in it: 4x Bulk cell, 4x Box of shells and 2x Shotgun shells. As you enter the area many monsters come teleporting in (and teleporting wildly around the place) but since there is a Megasphere there, too, you just have to survive and will walk out through the teleporter on top of the stairs good as new.